Typically, as shown in FIG. 1, a wireless communication system 10 comprises elements such as client terminal or mobile station 12 and base stations 14. Other network devices which may be employed, such as a mobile switching center, are not shown. In some wireless communication systems there may be only one base station and many client terminals while in some other communication systems such as cellular wireless communication systems there are multiple base stations and a large number of client terminals communicating with each base station.
As illustrated, the communication path from the base station (BS) to the client terminal direction is referred to herein as the downlink (DL) and the communication path from the client terminal to the base station direction is referred to herein as the uplink (UL). In some wireless communication systems the client terminal or mobile station (MS) communicates with the BS in both DL and UL directions. For instance, this is the case in cellular telephone systems. In other wireless communication systems the client terminal communicates with the base stations in only one direction, usually the DL. This may occur in applications such as paging.
The base station with which the client terminal is communicating is referred to as the serving base station. In some wireless communication systems the serving base station is normally referred to as the serving cell. While in practice a cell may include one or more base stations, a distinction is not made between a base station and a cell, and such terms may be used interchangeably herein. The base stations that are in the vicinity of the serving base station are called neighbor cell base stations. Similarly, in some wireless communication systems a neighbor base station is normally referred to as a neighbor cell.
Quadrature Amplitude Modulation (“QAM”), Phase Shift Keying (“PSK”), Binary PSK (“BPSK”), and Quadrature PSK (“QPSK”) are some of the commonly used modulation techniques in digital communication systems. The set of all symbols and their arrangement in a modulation technique is referred to as constellation.
The reference phase and amplitude of the modulation constellation are required at the receiver to estimate the symbols sent by the transmitter. In general, the phase and amplitude of the constellation show random variations due to many channel impairments such as fading, frequency response of the channel, frequency offset, timing offset, etc. In coherent detection at the receiver, the reference phase and amplitude of the modulation constellation may be obtained from reference symbols that may be transmitted along with the data symbols. In non-coherent detection, previously detected symbols may be used as reference symbols for detecting current symbols. Coherent detection may provide superior performance than non-coherent detection. The overhead in terms of the bandwidth and power allocated for transmitting reference symbols is justified by the improved performance. The process of estimating the phase and amplitude of the channel to demodulate the received symbol is referred to as channel estimation. The process of compensating the effect of the random phase and amplitude variation by using the estimated channel conditions is referred to as equalization.
Orthogonal frequency division multiplexing (OFDM) is used in many digital communication systems. In OFDM a large number of closely spaced orthogonal subcarriers is used to transmit data as shown in FIG. 2. The data are divided into several parallel data streams one for each subcarrier. Each subcarrier is modulated with a conventional modulation scheme such as QAM, PSK, BPSK, or QPSK, at a low symbol rate while maintaining total data rate similar to single carrier modulation schemes in the same bandwidth. The frequency spacing between two adjacent subcarriers is referred to as subcarrier spacing and it is denoted by Δf. The rate at which the individual subcarriers is modulated is referred to as symbol rate. The collection of all the subcarriers is referred to as an OFDM symbol. The OFDM symbol rate is the same as the data symbol rate on each individual subcarrier. The OFDM symbol duration is denoted by Tu. The OFDM signal is generated in frequency domain and then converted to time domain using an inverse Fast Fourier Transform (FFT). An OFDM signal over one symbol duration is referred to as an OFDM symbol in both time domain and frequency domain.
In case of a time dispersive channel the orthogonality between the subcarriers of an OFDM signal may be lost. The reason for this loss of subcarrier orthogonality is that the OFDM symbol boundary for one path will overlap with the symbol boundary of a different path, as illustrated in FIG. 3. As a consequence, in case of a time dispersive channel there will be inter-symbol interference within a subcarrier and interference between subcarriers.
Cyclic prefix insertion is typically used in OFDM to address the loss of orthogonality in time dispersive channels and to make an OFDM signal robust to time dispersion on the radio channel. As illustrated in FIG. 3, cyclic prefix insertion is performed by copying the last portion of the OFDM symbol and inserting it at the beginning of the OFDM symbol. Cyclic prefix insertion makes an OFDM signal robust to time dispersion as long as the span of the time dispersion does not exceed the length of the cyclic prefix.
At the receiver side, the samples corresponding to cyclic prefix are discarded before FFT processing to convert the received time domain signal to frequency domain. Assuming a sufficiently large cyclic prefix, the linear convolution of a time dispersive radio channel will appear as a circular convolution during the OFDM symbol interval Tu. The combination of OFDM modulation (inverse FFT processing), a time dispersive radio channel, and OFDM demodulation (FFT processing) can then be seen as a frequency domain channel as illustrated in FIG. 4, where the frequency domain channel taps h0, . . . , hN-1 can be directly derived from the channel impulse response, where N is the number of used subcarriers in an OFDM symbol
The output rk of the kth subcarrier at the receiver in FIG. 4 is the transmitted modulation symbol xk scaled and phase rotated by the complex frequency domain channel tap hk and impaired by noise nk. For coherent detection of the transmitted symbol, the receiver may multiply rk with the complex conjugate of the estimated channel, ĥk, as illustrated in FIG. 4. This is often referred to as a one tap frequency domain equalizer being applied to each received subcarrier.
To perform data demodulation, the receiver has to estimate the frequency domain channel taps h0, h1, . . . , hN-1. The frequency domain channel taps may be estimated by using known reference symbols, which may be inserted by the transmitter at regular intervals within the OFDM time-frequency grid, as illustrated in FIG. 5. Each basic element in the grid is referred herein as a Resource Element (“RE”) and the REs used for Reference Symbols (“RS”) are often referred to as pilot symbols or RS REs. The subcarrier on which the pilot symbol is transmitted is referred to as a pilot subcarrier. The terms pilot, pilot subcarrier, and pilot symbol are used interchangeably herein.
After converting an OFDM symbol into a frequency domain OFDM symbol the REs corresponding to the RS are used to estimate the channel since the modulation sequence for the reference symbols is known. After estimating the channel at the RS RE position, the channel estimation for the non-RS REs is performed using existing techniques known in literature, for example, a minimum mean square error (MMSE).
It is known that the timing position of the FFT window at the receiver must align exactly with the actual OFDM symbol boundary for optimum performance. Any timing error in positioning the FFT window relative to the true OFDM symbol boundary will appear as a linear phase distortion in the frequency domain. An example of this distortion is shown in FIG. 6. An adverse effect of the timing position error for the FFT window is that the correlation that exists between the adjacent subcarriers may be altered due to the linear phase distortion. The basis for the conventional channel estimation techniques such as MMSE is the correlation between the adjacent subcarriers. If the correlation between the adjacent subcarriers is altered, the quality of the channel estimation may be diminished. In mobile wireless communication systems, as the client terminals may be mobile, their position relative to the serving cell may change continually. The change in position may lead to changes in propagation delays and the reference timing as observed by the client terminal. A method and apparatus are disclosed that enable channel estimation that is robust against the timing errors.